1. Field of the Invention
The present invention relates to solid-state imaging apparatus of the CCD (charge coupled device) type, and in particular to a CCD type solid-state imaging apparatus preferable for reducing power consumption and a manufacturing method for the same.
2. Description of the Related Art
For example, as described in JP-A-02-62170, CCD type solid-state imaging apparatus comprises a vertical transfer path (VCCD) for reading signal charges accumulated in a photoelectric conversion part in accordance with the light receiving quantity and transferring the signal charges in a first direction and a horizontal transfer path (HCCD) for receiving the signal charges transferred by the vertical transfer path, transferring the signal charges in a second direction, and outputting the signal charges from the solid-state imaging apparatus.
FIG. 17A is a partial schematic cross-sectional view of a related art horizontal transfer path (HCCD). FIG. 17B is a partial schematic cross-sectional view of a related art vertical transfer path (VCCD). Both the horizontal transfer path and the vertical transfer path include transfer electrodes formed on a semiconductor substrate 1 via a gate insulating film 2. The illustrated example shows a two-layer transfer electrode. On the horizontal transfer path (FIG. 17A), a plurality of first layer transfer electrode films 3 composed of polysilicon are formed apart from each other on the insulating film 2. An oxidized insulating film 4 is formed on the surface of the first layer transfer electrode film 3 and second layer transfer electrode films 5 composed of polysilicon are formed so as to fill gaps between the first layer transfer electrode films 3. On the surface of the second layer transfer electrode film 5 is formed an oxidized insulating film 6.
On the vertical transfer path (FIG. 17B) also, a plurality of first layer transfer electrode films 8 are formed apart from each other on the gate insulating film 2. An oxidized insulating film 9 is formed on the surface of the first layer transfer electrode film 8 and second layer transfer electrode films 10 composed of polysilicon are formed so as to fill gaps between the first layer transfer electrode films 8. On the surface of the second layer transfer electrode film 10 is formed an oxidized insulating film 11.
The horizontal transfer path and the vertical transfer path of related art CCD type solid-state imaging apparatus are manufactured in the same manufacturing process, so that the interlayer insulating film 4 of the horizontal transfer path has the same film thickness as the interlayer insulating film 9 of the vertical transfer path.
In case CCD type solid-state imaging apparatus is driven, signal charges in a single row of photoelectric conversion elements arranged in parallel with the horizontal transfer path are transferred to the horizontal transfer path. The horizontal transfer path is driven for transfer so as to output all signals corresponding to the signal charges in the single row from the solid-state imaging apparatus. Signal charges in the next row of photoelectric conversion elements are then transferred to the horizontal transfer path. This sequence is repeated. It is thus necessary to perform transfer driving of the horizontal transfer path at a higher speed than that of the vertical transfer path.
The horizontal transfer path that is typically driven at a high frequency of 12 to 36 MHz is one of the most power-consuming components of solid-state imaging apparatus. Power consumption of the horizontal transfer path is proportional to the square of the driving voltage of the horizontal transfer path (typically 0V for a low level and 3.3V for a high level). Reduction of the driving voltage can reduce the power consumption of the CCD type solid-state imaging apparatus.
Experiments by the inventor have proven that it is possible to reduce the driving voltage of the horizontal transfer path by providing a thinner interlayer insulating film 4 shown in FIG. 17A. The problem is that, in the manufacturing process of related art CCD type solid-state imaging apparatus, the interlayer insulating film 4 of the horizontal transfer path has the same film thickness as the interlayer insulating film 9 of the vertical transfer path, so that the withstand voltage across the electrode films 8 and 10 of the vertical transfer path that requires a larger driving voltage (typically −8V for a low level and 0V for a high level) limits the reduction of the thickness of the interlayer insulating film 4.